Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
An ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line's resolution is a result of the directivity of the associated transmit and receive beam pair. In particular, because of the beamforming time delays applied to both the transmitted and received signals for each transducer element, backscattered signals from tissue along the line of the steering angle and at the transmit focal zone position add up coherently and produce large composite beam sums, while back-scatter from tissue off the beam axis and out of the transmit focal zone add incoherently and produce a relatively smaller beam sum.
Tissue types and anatomical features are most easily differentiated in an ultrasound image when they differ in image brightness. Image brightness on conventional medical ultrasound imaging systems is a function of the amplitude of the receive beamformed signal, i.e., after coherent summation of the delayed receive signals on each transducer element. More precisely, the logarithm of the amplitude of the beamformed signal is displayed, with user-adjustable gain and contrast, and perhaps a choice of a handful of gray-scale mapping tables.
Unfortunately, very strong signals off of the ultrasound beam steering direction can often produce a signal which, when added incoherently, still produces a large enough composite signal to degrade or mask the coherently summed smaller tissue signals. In addition, structures in the body, such as varying layers of muscle and fat, can produce ultrasound time-of-flight variations that cause tissue signals along the steered direction to add incoherently, thereby degrading the resolution in the display image of those tissue structures.
There is a need for an imaging technique that would detect when the acoustic data acquired by the imager is incoherent and then adaptively suppress that incoherent data to mitigate degradation of the displayed image.